A physical analysis of the severe 2013/2014 cold winter in North America

Abstract

AbstractThe severe 2013/2014 cold winter has been examined in the context of the previous 55 winters using the National Centers for Environmental Prediction reanalysis data for the period 1960–2014. North America is dominated by pronounced cold anomalies over the Great Plains and Great Lakes in December 2013 and February 2014 but exhibits an east‐west contrast pattern with warm anomalies over most of the North American West in January 2014. A relevant temperature index, defined as land surface temperature anomalies averaged over (40°–60°N, 105°–80°W), reveals a warming trend as well as interannual variability with a significant power peak of 6.0 years. While 2013/2014 was the second coldest winter during 1960–2014, it is the coldest one in the linearly detrended series, with a negative anomaly of 2.63 standard deviations. This indicates that the long‐term warming has made the 2013/2014 winter less severe than it could have been. The temperature and circulation variability in association with the zonally symmetric variability of the polar vortex projects weakly on the corresponding anomalies in the 2013/2014 winter, whereas the variability associated with the principal mode of North American surface temperature projects strongly on the corresponding anomalies in the winter. This mode is associated with a sea surface temperature (SST) pattern of significant anomalies over the North Pacific and North Atlantic middle and high latitudes. The anomalous atmospheric circulation shows an anticyclonic anomaly over the Gulf of Alaska‐Bering Sea and a cyclonic anomaly downstream over North America. It bears resemblance to the North Pacific Oscillation/Western Pacific pattern and drives the SST in the North Pacific. Over western‐central Canada and the northern U.S., below‐average heights are associated with above‐normal precipitation, implying enhanced upward vertical motion and variation of local cloud forcing, leading to a variation of the surface energy budget dominated by surface longwave radiation anomalies. Over North America, there is less downwelling longwave radiation at the surface when the atmosphere is cold, which is offset by the corresponding reduction in outgoing longwave radiation.